Smoke Simulation has weird artifacts and looks low res
You can:


Increase the domain's Division value (this will change how the smoke simulates, interacts and looks and will increase simulation time)
Turn on Smoke High Resolution (this will introduce noise into the simulation to add detail, more info in manual).
Check if the domain material has Cubic set for it's voxel interpolation (should be by default):




Volumetric domains are like raster images but in 3D. Imagine rendering a low resolution texture close to camera on a FullHD render - no matter what you do, even if you go to 4K, the texture would still render as ugly blob.